1. Field of the Invention
This invention relates to a method of fabricating a surface probing device and the probing device produced thereby, and more particularly, to a method of fabricating a silicon tip supported by a silicon nitride cantilever for performing surface analysis on a sample.
2. Description of Related Art
Surface analysis methods have advanced to achieve atomic resolution using a probing tip of a surface probing device having an apex of atomic dimensions. The probing tip is usually a tapered silicon structure, often referred to as a stylus, with a base attached to a cantilever arm and a sharp apex that interacts with the surface being probed. More particularly, the parts of the surface probing device include a stylus, a cantilever arm and a mounting section. In addition, the surface probing device may have an electrical connection from the stylus, through the cantilever arm, and to external circuitry for monitoring surface characteristics in a particular mode of operation. Moreover, the probe device may also have a reflective coating on the cantilever arm to accommodate, for example, optical detection techniques. In general, the electrical connection and the reflective coating provide different ways to measure the response of the stylus apex to the surface being analyzed.
An apparatus that uses a surface probing device for surface analysis typically involves a scanning process. During the scanning process, the stylus apex responds to surface characteristics. The response is monitored and generally held constant through a feedback system that causes a slight change in the cantilever arm position. Two notable examples where these general principles apply are scanning tunneling microscopy (STM) and atomic force microscopy (AFM).
In STM, a stylus apex of atomic dimensions on a cantilever arm follows the contour of a sample surface. Electrons tunnel through a near-field vacuum between the conductive apex of the stylus and a conducting sample creating a tunneling current. The tunneling current is very sensitive to changes in the distance between the stylus apex and the conductive sample surface. A feedback system is used to monitor and control the tunneling current at a constant value. Optionally, an optical detection techniques such as interferometry or laser beam deflection can be used to measure the resultant cantilever arm deflection during scanning.
AFM uses a stylus that is mounted on a cantilever arm that has a small spring constant and scans a surface such that repulsive inter-atomic forces between the surface and the stylus apex cause deflections in the cantilever arm position. Again, a feedback system is used to monitor and control the forces between the tip and sample, and an optical detection technique such as interferometry or laser beam deflection are used to measure the resultant cantilever arm deflection during the scanning process. In AFM, different modes of operation may be employed. See, for example, U.S. Pat. No. 6,189,374, filed Mar. 29, 1999, assigned to the present assignee, and entitled Active Probe For An Atomic Force Microscope And Method Of Use Thereof.
Several methods for fabricating surface probing devices with a stylus and a cantilever arm have been reported. Bothra et al., U.S. Pat. No. 5,540,958, describe a method for making a stylus on a cantilever arm by first etching a silicon wafer with a mask to produce protruding shapes of a predetermined size and then depositing a second layer, such as silicon oxide, by electron cyclotron resonance. Shimada et al., in U.S. Pat. No. 5,546,375 describe making a stylus by forming a recessed cavity in a silicon wafer. The cavity is then used to define the structure of the stylus. In U.S. Pat. No. 5,399,232, Albrecht et al. describe a method of fabricating a cantilever arm and stylus again by forming a depressed area in a silicon wafer and using the depressed area to define the stylus shape. In U.S. Pat. No. 5,581,083, Majumdar et al. describe a method for producing a hole at the apex of a stylus. The method uses a voltage applied to a metal coated tip causing evaporation of the metal coating and exposing the underlying silicon apex. Manalis et al., in U.S. Pat. No. 6,156,216, describe a silicon nitride cantilever with a silicon tip but provide no way for making a tip useable for probe microscopy, nor a means to control the characteristics of the silicon tip while removing the silicon nitride covering.
As noted, the combination of a stylus and a cantilever arm is important for many modern surface probing methods. In addition, each method of analysis typically requires a stylus and a cantilever arm with properties tailored to the application at hand. A significant drawback in this regard is that known methods to fabricate silicon styluses supported by cantilever arms include making the cantilever arms from silicon using an etching process. One difficulty that can arise in fabricating surface probing devices with silicon cantilever arms is that the thickness of silicon is difficult to control by etching. Another drawback is that it is beneficial for some applications such as thermal measurement and electrical measurement to make surface probing devices that contain an electrically isolated stylus which can be connected to external circuitry through a conductive metal deposited on the cantilever arm.
Therefore, the field of fabricating such surface probing device is in need of a method for fabricating corresponding styluses and cantilever arms in which the thickness of the cantilever arm is easy to control during the fabrication process and which yields an electrically isolated silicon tip. Moreover, it is important that the stylus be extremely sharp or, alternatively, small. A typical state of the art silicon AFM probe tip has a radius of curvature smaller than 15 nm. However, cantilevers with lower spring constants, such as silicon nitride cantilevers typically have styluses with radii of curvature greater than 25 nm. This is due to the fact that silicon nitride tips are typically molded, thus yielding tips that are necessarily less sharp. Contrary to such silicon nitride tips, silicon tips can be readily sharpened via an oxidation step. Therefore, in applications that require the low force afforded by the silicon nitride cantilever, resolution due to stylus size must be sacrificed.
In sum, a method that workably combines a silicon tip with a silicon nitride cantilever is thus desired to achieve the benefits of both systems, i.e., a low force silicon nitride cantilever and a sharp silicon tip.